varm/stdm and stats dimension arguments

Largely addresses #2265 by renaming the known-mean variants of `var` and `std` to `varm` and `stdm`. In their place are `std(v[, region])`, `var(v[, region])`, and also `mean(v[, region])`. 

This pull request also drops the 'corrected' flag for these functions, as suggested by @nolta.